Bent metal member and a method for its manufacture

ABSTRACT

A bent metal member has a body formed from a tubular metal material and has a shape which is bent two-dimensionally or three-dimensionally. The surface of the body has a Zn-based coating which contains an η phase and which satisfies the following condition: coating weight: at most 90 g/m 2  per side, Fe content: 8-35 mass %, and a surface roughness Ra prescribed by WS B 0601: at most 2.0 μm. The bent metal member can have a closed cross section, an open cross section, or a shaped cross section.

TECHNICAL FIELD

This invention relates to a bent metal member and a method for itsmanufacture. Specifically, the present invention relates to ahigh-strength bent metal member which is manufactured by bendingaccompanied by heat treatment of a zinc-based coated metal material andwhich has a high strength and excellent post-painting corrosionresistance (corrosion resistance after painting) which make it suitablefor use as an automobile part. This invention also relates to a methodfor its manufacture.

BACKGROUND ART

Out of concern for the global environment, there has been a demand inrecent years that steel structural members for automobiles have a lightweight and a high strength. Techniques for increasing impact energyabsorbing ability are also being developed in order to increase thesafety of automobile bodies in collisions.

For example, metal tubes such as steel tubes are used as beams forreinforcing doors in order to increase safety during side impacts. Theimpact energy absorbing ability of door reinforcing beams is increasedby giving a metal tube a bent shape. The impact energy absorbing abilityof a center pillar reinforcing member is improved by optimizing theshape and curvature of the reinforcing member. Various techniques arebeing employed for bending steel tubes (in this description, steel tubesinclude not only usual steel tubes such as welded steel tubes but alsomembers produced by forming a steel sheet into a tubular shape) into anoptimal shape for automotive parts.

There is a strong demand for an increase in the tensile strength ofautomotive parts in order to decrease the weight of automotive bodies.High-strength steel materials having a high strength level such as, forexample, a tensile strength of at least 780 MPa or even at least 900MPa, which is completely different from that used in the past, are beingwidely used.

It is difficult to carry out bending of a steel material made from ahigh tensile strength steel in a cold state. Furthermore, bending of asteel material made from a high tensile strength steel in a hot state isalso difficult because non-uniform strains develop in the steelmaterial, which causes the shape of a bent product to fluctuate, andshape retention m is inadequate. Furthermore, there is a strong demandfor the development of bending techniques which can manufacture withhigh accuracy a steel member having various bent shapes such as a bentshape in which the bending direction varies two-dimensionally orthree-dimensionally.

In PCT/JP2006/303220, the present applicant disclosed an apparatus formanufacturing a bent member. That apparatus (a) supports a materialbeing worked at two locations spaced from each other in the axialdirection of the material so that the material can move in its axialdirection, (b) it feeds the material being worked in the axial directionof the material, (c) it rapidly heats the material being worked with aninduction heating coil between the two locations where the materialbeing worked is supported to a temperature at which plastic working ofthe material is possible and at which the crystal grains of the materialdo not coarsen, (d) it cools the heated material being worked with acooling device, and (e) it two-dimensionally or three-dimensionallyvaries the position of a movable roller die which supports the materialbeing worked at the downstream in the feed direction of the materialbeing worked of the two locations where the material being worked issupported whereby a bending moment is imparted to the high temperatureportion of the material being worked and this portion is plasticallydeformed.

It is economical to carry out induction heating of the material beingworked by that apparatus in air. Steel members used in automotive partsbasically undergo chemical conversion treatment and electrodepositioncoating. In order to increase corrosion resistance, automobile parts aremade from a zinc-based coated steel material.

Accordingly, by using zinc-based coated steel as the material beingworked by that apparatus, it is not only possible to prevent oxidationof the material being worked but it is also possible to manufacture abent member or a quench hardened member having excellent corrosionresistance. Therefore, it is possible to widely employ a material whichhas been worked by that apparatus in automobile parts.

However, when a zinc-based coated steel material is heated to at leastthe A₁ transformation point or even to at least the A₃ transformationpoint, the performance of the coating layer may deteriorate. This isbecause the vapor pressure of Zn suddenly rises with an increase intemperature, as exemplified by the fact that the vapor pressure is 200mm Hg at 788° C. and 400 mm Hg at 844° C. As a result, Zn may vaporizeduring a heating step.

A manufacturing method for a member which is strengthened by highfrequency quench hardening is disclosed in Patent Document 1 as atechnique for heat treatment of a zinc-based coated steel sheet. In thatmethod, a galvanized steel sheet for high frequency hardening is heatedto a quench hardening temperature of at least the Ar_(a) point and atmost 1000° C. and then cooled with the interval from the start ofheating until cooling to 350° C. being restricted to at most 60 seconds.Patent Document 1 discloses that even if a hot-dip zinc-based coatedsteel sheet having a base steel sheet for quench hardening is used as amaterial to be strengthened by high frequency hardening and highfrequency quench hardening is carried out on a portion which is to beincreased in strength, a coating layer remains on the quench hardenedportion, and the Fe concentration in the coating layer can be restrictedto at most 35% (in this description, unless otherwise specified, % meansmass %). Therefore, a member for automobiles having excellentcoatability and corrosion resistance is obtained.

Patent Document 2 and Patent Document 3 disclose methods in which amaterial being worked made of a zinc-based coated steel sheet is heatedfor several minutes at 700-1000° C., for example, and hot pressing iscarried out.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 2000-248338 A

Patent Document 2: JP 2001-353548 A

Patent Document 3: JP 2003-73774 A

SUMMARY OF THE INVENTION Problem which the Invention is to Solve

The nature of a coating layer which is desired to exist on the surfaceof a steel member after heat treatment vary in accordance with the useof the steel member. However, a coating layer having a pure zinc phase(η-Zn phase), which electrochemically has a base potential, isadvantageous when it is desired for a coating layer to have goodsacrificial corrosion protecting properties. Patent Document 1 disclosesonly that the Fe content of the coating layer of a heat treated materialis at most 35%.

In the methods disclosed in Patent Documents 2 and 3, if the residencetime at a heating temperature is long, mutual diffusion between theelements of the coating layer and the steel in the substrate progresses.As a result, an intermetallic compound phase having a high Feconcentration or a solid solution phase in which Zn is dissolved in α-Feis formed on the surface of the heat-treated steel material.

Means for Solving the Problem

The method disclosed in above-mentioned International Application No.PCT/JP 2006/303220 is a novel process in which plastic deformation isimparted to a material being worked during heating without the materialcontacting a die, rapid cooling is then performed, and then the materialpasses through a movable roller die. That method is different from themethods disclosed in Patent Documents 2 and 3 (hot pressing in which amaterial being worked is heated and then interposed between a die and apress to undergo plastic deformation, and then cooled in the die) or themethod disclosed in Patent Document 1 (so-called high frequency heatingand hardening in which a material being worked undergoes quenchhardening treatment by simply being heated and cooled without undergoingplastic deformation).

If the method disclosed in International Application No.PCT/W2006/303220 is possible to manufacture a heat treated materialhaving a coated layer with an η-Zn phase close to pure zinc on itssurface even after heat treatment, it can provide a desired bent metalmember and a method for its manufacture.

For this purpose, it is conceivable to use a steel material having apure zinc coating (specifically, a hot-dip galvanized steel material oran electrogalvanized steel material) as a material to undergo heattreatment. However, a pure zinc phase has a low melting point, so duringheating, the surface becomes a liquid phase, and defects such asdripping easily develop in the appearance of the steel material aftercooling. In addition, it is difficult to eliminate the above-describedvaporization of zinc.

Even when a steel material having a Fe—Zn based alloy coating with an Fecontent of around 10% (specifically, a galvannealed coating) is used asa material to be heat treated, it is thought that a liquid phase isformed when the steel material is heated to a temperature exceedingapproximately 660° C. at which the Γ phase (Fe₃Zn₁₀) decomposes in aphase diagram.

However, when such a steel material is rapidly heated and thenimmediately thereafter rapidly cooled, the period of residence in aheating temperature range including both heating and cooling stages isextremely short, and the time for which diffusion progresses is short.Therefore, in the solidification stage, it is thought that the liquidphase solidifies while discharging an intermetallic compound phasehaving a high Fe content which has a high melting point, and as aresult, solidification takes place such that an η phase having anextremely low Fe content is present. Since the decomposition temperatureof a Γ phase is higher than the melting point of a pure zinc phase, itis thought that the entirety of the coating layer does not become Zn,and it becomes difficult for dripping or consumption by vaporization ofthe coating layer to develop.

By employing such rapid heating and rapid cooling conditions in themanufacturing method for a bent member disclosed in InternationalApplication No. PCT/JP2006/303220, it is possible to continuously carryout working and heat treatment without contacting the surface of a steelmaterial in a liquid state with a die such as takes place in hotpressing. The present invention is based on such findings.

The present invention is a method of manufacturing a bent metal membercharacterized by supporting the outer surface of a tubular metalmaterial having a Zn—Fe alloy coating layer which satisfiesbelow-described Condition 1 on its surface at two locations spaced inthe axial direction of the metal material so that the material can movein the axial direction, heating the metal material at a position betweenthe two locations while feeding it in its axial direction to atemperature range of at least the Ac₃ point and preferably at least theAc₃ point to at most 9.5×10²° C. at a heating speed of at least 3.0×10²°C. per second and cooling it after holding the temperature of the metalmaterial such that the length of time for which the surface of the metalmaterial is at a temperature of 8.0×10²° C. or higher is at most 2seconds, and two-dimensionally or three-dimensionally varying one of thetwo locations which is located downstream in the feed direction of themetal material, thereby imparting a bending moment to the heated portionof the metal material to manufacture a bent metal member having on itssurface a Zn-based layer which contains an η phase and which satisfiesthe following Condition 2:

(Condition 1)

-   -   Coating weight: 30-90 g/m² per side    -   Fe content: 8-20%    -   Surface roughness Ra prescribed by JIS B 0601: At most 0.8 μm;

(Condition 2)

-   -   Coating weight: At most 90 g/m² per side    -   Fe content: At most 8-35%    -   Surface roughness Ra prescribed by JIS B 0601: At most 2.0 μm.

From another standpoint, the present invention is a bent metal memberhaving a body which is made from a tubular metal material and which hasa shape which is bent two-dimensionally or three-dimensionally,characterized in that the surface of the body has a Zn-based layer whichcontains an ₁₁ phase and which satisfies above-described Condition 2.Examples of the bent metal member are members having a closed crosssection, an open cross section, or a shaped cross section.

Effects of the Invention

According to the present invention, a bent metal member which isobtained by carrying out bending accompanied by heat treatment of azinc-based coated metal material and which is suitable for use as acomponent of automobiles and the like due to having a high strength andexcellent post-painting corrosion resistance and a method for itsmanufacture can be provided.

Specifically, according to the present invention, it is possible to usea zinc-based coated steel material as the material being worked evenwhen carrying out in air the method disclosed in InternationalApplication No. PCT/JP2006/303220, namely, a method of manufacturing abent member by supporting a material being worked at two locationsspaced in the axial direction of the material being worked so that thematerial being worked can move in its axial direction, rapidly heatingand cooling the material being worked at a position between the twolocations where the material being worked is supported while feeding thematerial in its axial direction, and two-dimensionally orthree-dimensionally varying the position of one of the two locationswhich is located downstream in the feed direction of the material beingworked to impart a bending moment to the heated portion of the materialbeing worked.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is an explanatory view showing an example of the structure of amanufacturing apparatus for carrying out a method of manufacturing abent metal member.

FIG. 2 shows graphs of heating patterns of test materials.

FIG. 3 shows graphs of the X-ray diffraction strength of test materials.

FIG. 4 shows SEM images of the surface and cross section of testmaterials.

EXPLANATION OF SYMBOLS

1 material being worked

1 a zinc-based coated steel material

1 b zinc-based coated heat treated steel material

2 support rolls

3 feed device

4 movable roller die

5 induction heating coil

6 cooling device

EMBODIMENTS OF THE INVENTION

Below, preferred embodiments of a bent metal member and a method for itsmanufacture according to the present invention will be explained whilereferring to the accompanying drawings.

The present invention is highly practical in that a heat treated steelmember is obtained by carrying out hot bending and quench hardening atapproximately the same time on a zinc-based coated steel material whichis in the form of a steel tube or the like manufactured from a basesteel sheet (sometimes referred to below as a starting sheet). A steeltube includes not only a welded steel tube but also one which is formedinto a tubular shape by roll forming or the like. The cross-sectionalshape of the steel tube is not limited and may be circular, polygonal,or the like.

The present invention will be explained in the order of the steps in abending process.

(i) Steel Material Before Heating (Starting Sheet)

(a) Coating Layer (Coating Weight, Fe Content)

A tubular metal material which is used as a material being worked israpidly heated so that, as described below, the peak metal temperatureis at least the Ac₃ point (the Ac₃ point depends upon the type of steel,but for practical purposes it is at least approximately 850° C.). It isconceivable that a slight amount of Zn evaporates during the heatingprocess. It is preferable to leave a coating weight of at least 5 g/m²in order to guarantee adequate corrosion resistance after heattreatment. It is more preferable to leave a coating weight of at least10 g/m² and still more preferable to leave a coating weight of at least20 g/m². For this purpose, the starting sheet prior to workingpreferably has a certain amount of coating adhered thereto.

If the coating weight is too great, the amount of liquid phase increasesduring heating, and defects in the appearance such as so-called drippingcan easily develop in the steel material after cooling. It is desirableto increase the melting point of the coating layer by previously formingan alloy coating layer.

From these standpoints, the coating weight on a material being worked inthe faun of a tubular metal material is preferably 30-90 g/m² and a morepreferred coating weight is 40-70 g/m². The coating layer is preferablya Zn—Fe alloy coating layer (preferably a galvannealed coating layer)containing 8-20% of Fe rather than pure zinc coating.

The zinc coating layer may contain Al. A preferred Al content is at most0.45%. If the Al content of the coating layer is too high, the Fe—Znalloying reaction during the heating step becomes nonuniform and thesurface roughness after cooling can easily become too high.

(Surface Roughness of the Coating Layer)

The surface of a bent metal member is preferably smooth. The surfaceroughness affects the ability of the member to undergo degreasing andchemical conversion treatment. For example, it is thought that when arust preventing oil is applied to a heat treated material, the rustpreventing oil enters into irregularities formed on the surface of theheat treated material. It is advantageous for the surface of a bentmetal material to be smooth in order to guarantee good degreasingability. It is advantageous for a bent metal member to have a smoothsurface from the standpoints of the appearance after painting andpost-painting corrosion resistance (resistance to blistering of apainted coating and the like).

A low surface roughness of a bent metal member after heat treatment caneasily be achieved by making the surface roughness of the coating layeron the metal material before heat treatment as low as possible. Inparticular, when a galvannealed steel sheet is used as a starting sheet,microscopic non-uniformities in alloying of Fe—Zn at the time ofalloying treatment affect the surface roughness, so the surfaceroughness before heat treatment is preferably made as low as possible. Atarget surface roughness of the metal material is preferably a surfaceroughness Ra prescribed by JIS B 0601 of at most 0.8 μm. Examples ofmethods of decreasing the surface roughness Ra are skin pass rolling ofa galvannealed steel sheet and decreasing the surface roughness by thebelow-described tube forming or roll forming or the like.

It is expected that the present invention can exhibit an advantageouseffect on corrosion resistance by forming a coating having an η phase.As stated above, because a coating formed by a heat treatment processaccording to the present invention has undergone the formation of liquidphase, surface irregularities are smoothed and the appearance afterpainting and post-painting corrosion resistance (difficulty of formingblisters in the painted layer and the like) are improved.

(b) Steel Composition

The base steel of a galvannealed steel material used in the presentinvention preferably has a steel composition which is quench hardenableeven when heated to a temperature of not higher than 1050° C. By havingsuch a steel composition, working can be easily carried out in atube-forming stage and the like on a steel in its low-strength state,and then the strength can be increased by subsequent quench hardening toobtain a desired high strength. Such a steel composition is exemplifiedby a steel composition of a hardenable steel which comprises C:0.1-0.3%, Si: 0.01-0.5%, Mn: 0.5-3.0%, P: 0.003-0.05%, S: at most 0.05%,Cr: 0.1-0.5%, Ti: 0.01-0.1%, Al: at most 1%, B: 0.0002-0.004%, N: atmost 0.01%, a remainder of Fe and impurities, and if necessary, one ormore of Cu: at most 1%, Ni: at most 2%, Mo: at most 1%, V: at most 1%,and Nb: at most 1%.

(c) Forming Before Heat Treatment

Before carrying out the below-described bending, a starting sheet asdescribed above is formed into a shape such as a steel tube. A typicalformed shape is a UO tube which undergoes UO framing and then seamwelding, but the present invention is not limited to a UO tube.Depending on the use and bent shape, members having a closed crosssection with a cross-sectional shape such as a circle, a rectangle, atrapezoid, or the like, members with an open cross section (channels)manufactured by roll forming or the like, and members with a shapedcross section (channels) manufactured by extrusion can be used as theformed shape. Tapered steel members having a cross-sectional area whichcontinuously varies can also be used as a formed shape.

(ii) Heating—Hot Bending—Cooling

The outer surface of a tubular metal material having a Zn—Fe alloycoating layer as described above is supported at two locations spaced inthe axial direction of the metal material so that the material can movein its the axial direction. The metal material is fed in its axialdirection.

The metal material is heated at a position between the two locations toa temperature range of at least the Ac₃ point and preferably atemperature range from the Ac₃ point to 950° C. at a rate of temperatureincrease of at least 300° C. per second. The metal material is held sothat the length of time for which its surface is at a temperature of800° C. or higher is most 2 seconds. The metal material is then rapidlycooled.

A bending moment is applied to the high temperature portion of the metalmaterial by two-dimensionally or three-dimensionally varying theposition of one of the two locations which is located downstream in thefeed direction of the metal material.

(a) Heating Pattern

(Heating Temperature, Cooling Speed, Holding Time)

The metal material is heated to a temperature range of at least the Ac₃point and preferably from the Ac₃ point to 950° C. The heating patternfor heating is a rate of temperature increase at the time of heating ofat least 300° C. per second, a holding time at which the surface of themetal material is at least 800° C. being at most 2 seconds, and acooling speed of at least 400° C. per second, for example.

By employing this heating pattern, an alloying reaction between thecoating layer and the steel is suppressed, and a coating containing an ηphase is formed after heating since rapid cooling takes place after aliquid phase is present on the surface at the time of heating.

(Peak Metal Temperature During Heating)

The peak metal temperature at the time of heating is preferably nothigher than 1150° C. If the heating temperature is too high, the amountof the coating remaining after cooling may become too small, and even ifa coating remains, the surface of the coating becomes coarse. It isthought that a high heating temperature increases the amount of a liquidphase present on the surface whereby there is an increased loss of thecoating due to vaporization and contact with a cooling medium.

As shown by the liquidus line in a Fe—Zn phase diagram, the temperatureat which an alloy consisting of Zn and approximately 10% of Fe entirelymelts is approximately 930° C. However, in the present invention,heating and cooling in this temperature range are rapid and the holdingtime is short. Therefore, it is thought that a coating remains even ifheating is carried out to a temperature higher than the meltingtemperature in an equilibrium state.

(iii) Bending

By heating the metal material with a heating pattern as described aboveand two-dimensionally or three-dimensionally varying the position of thedownstream of the two locations where the outer surface of the metalmaterial is supported so that the material can move in its axialdirection, a bending moment is imparted to the portion of the metalmaterial which is at a high temperature.

FIG. 1 is an explanatory view showing an example of the structure of amanufacturing apparatus 0 for carrying out a method of manufacturing abent metal member. In the example shown in FIG. 1, a metal material 1which is a material being worked is a round tube which is made ofzinc-based coated steel and which has a circular closed cross section.

As shown in FIG. 1, the outer surface of a zinc-based coated steelmaterial 1 a which is the material to be worked is supported at twolocations A and B spaced in the axial direction of the metal material 1so that it can move in its axial direction. The metal material 1 is fedin the axial direction. The metal material 1 is heated, held, and cooledwith the above-described heating pattern between the two locations A andB, and the position of location B is two-dimensionally orthree-dimensionally varied. In this manner, a bending moment is impartedto the portion of the metal material 1 which is at a high temperature,and this portion is plastically deformed.

For this purpose, the manufacturing apparatus 0 has a feed device 3 forthe metal material 1, two sets of support roll pairs 2, 2, an inductionheating coil 5, a cooling device 6, and a movable roller pair 4, 4disposed in this order in the feed direction of the zinc-based coatedsteel material 1 a. These components of the manufacturing apparatus 0will next be explained.

The support roll pairs 2, 2 rotate while contacting the outer surface ofthe zinc-based coated steel material 1 a which is being fed in its axialdirection. As a result, the support roll pairs 2, 2 support thezinc-based coated steel material 1 a so that it can move in its axialdirection. In the illustrated example, two sets of support roll pairs 2,2 are provided in the feed direction of the zinc-based coated steelmaterial 1 a.

The feed device 3 is disposed on the upstream side of the two sets ofsupport roll pairs 2, 2. The feed device 3 continuously orintermittently feeds the zinc-based coated steel material la in itsaxial direction.

The movable roller pair 4, 4 is disposed on the downstream side of thetwo sets of support roll pairs 2, 2. The movable roller pair 4, 4rotates while contacting the outer surface of the zinc-based coatedsteel material 1 a which is being fed in its axial direction and therebysupport the zinc-based coated steel material la so that it can move inits axial direction. The position of the movable roller pair 4, 4 can betwo-dimensionally or three-dimensionally varied. In the illustratedexample, the movable roller pair 4, 4 is rotatably mounted in anunillustrated casing.

The movable roller die includes the movable roller pair 4, 4. Themovable roller die has a vertical shifting mechanism, a shiftingmechanism for shifting to the left and right, a tilting mechanism fortilting in the vertical direction, and a tilting mechanism for tiltingto the left and right. The movable roller die also has a mechanism forforward and backward movement. With this structure, the movable rollerdie moves while three-dimensionally positioning the zinc-based coatedsteel material 1 a. By moving three-dimensionally, the movable rollerdie imparts a bending moment to the heated portion of the zinc-basedcoated steel material 1 a.

The induction heating coil 5 is disposed between the two sets of supportroll pairs 2, 2 and the movable roller pair 4, 4 and is spaced from theouter periphery of the zinc-based coated steel material 1 a. Theinduction heating coil 5 heats the zinc-based coated steel material 1which is being fed.

The cooling device 6 is disposed in the vicinity of the inductionheating coil 5. The cooling device 6 rapidly cools the zinc-based coatedmetal material 1 a which was rapidly heated by the induction heatingcoil 5.

When a bent metal member is manufactured using this manufacturingapparatus 0, the zinc-based coated metal material 1 a may undergobending (heat treatment) over its entire length in the axial direction,or only necessary portions of the length may undergo bending (heattreatment) by locally carrying out bending. For example, when a bentmetal member is a bumper reinforcing member for an automobile, it is notnecessary to carry out bending and quench hardening over its entirelength, and bending and quench hardening can be omitted at the endportions in the axial direction, for example.

In this manner, a bent metal member in the form of a zinc-based coatedheat treated metal member is manufactured. A Zn-based layer whichcontains an η phase is formed on the surface of the bent metal member.This layer has a coating weight of at most 90 g/m² per side, an Fecontent of 8-35%, and a surface roughness Ra prescribed by JIS B 0601 ofat most 2.0 μm.

It can be ascertained whether an η phase is present by X-ray diffractionfrom the surface. A heat treated member having this layer has a metallicluster with a green tint. Its external appearance is totally differentfrom the color (dark brown to black) of a heat treated material havingscale formed on steel.

In this manner, a bent metal member is manufactured by performingbending accompanied by heat treatment on a zinc-based coated metalmaterial. This bent metal member has a high strength and excellentpost-painting corrosion resistance, so it is suitable for use as acomponent for automobiles and the like.

Therefore, it is possible to use zinc-based coated steel as a materialbeing worked even when carrying out the method disclosed inInternational Application No. PCT/JP2006/303220 in air.

EXAMPLE 1

A galvannealed steel sheet having the steel composition shown in Table 1(sheet thickness of 2.3 mm, coating weight of 60 g/m², Fe content in thecoating layer of 13%) underwent UO forming and then laser welding toprepare a tube with an outer diameter of 31.8 mm, a wall thickness of2.3 mm, and a length of 2000 mm.

The surface roughness Ra of this tube was 0.56 μm on the outer surfaceand 1.44 μm on the inner surface.

TABLE 1 Calculated Steel type C Si Mn P S sol. Al N Ti Ac₃ Test 0.210.23 1.22 0.01 0.002 0.037 0.0028 0.028 791° C. material

Formula for calculating Ac₃: 910-203(C)^(1/2)+44.7Si−30Mn, wherein C,Si, and Mn indicate the mass % of these elements in the chemicalcomposition.

Using the thus-prepared tube as a material being worked, heating,holding, and cooling were carried out under predetermined conditionsusing the manufacturing apparatus 0 shown in FIG. 1 to obtain a testmaterial. Heating of the tube was carried out using a high frequencyheater. Cooling was carried out by water cooling from the outer surfaceof the tube using a water cooling device provided immediately after thehigh-frequency heating device.

Table 2 shows the heat treatment conditions (rate of temperatureincrease, holding time, peak metal temperature) and the results ofinvestigation of the surface condition.

The conditions were set by adjusting the output of the high frequencyheating device, the feed speed of the steel tube, and the like, and thetemperature was measured using a thermocouple attached to the outersurface of the tube. The rate of temperature increase was calculatedfrom the time required for heating from 200° C. to the peak metaltemperature reached, and the holding time was the holding time in atemperature range of 800° C. or higher. The surface condition wasinvestigated with respect to the following categories.

1) Visual Observation

The surface of the steel after heat treatment was visually observed toevaluate the extent to which a coating layer remained.

Specimens having a metallic luster with a greenish tint were evaluatedas good (CIRCLE), specimens in which the color of the coating turnedblack but scale was not observed were evaluated as good (TRIANGLE), andspecimens in which the color became dark brown or black, the coatingdisappeared, and scale developed were evaluated as poor (X).

2) Observation of Cross Section

For some of the heat treated materials, the condition of the surfacelayer was evaluated by observing a cross section with a SEM.

3) Phase Structure of Coating (Presence or Absence of an η Phase)

Specimens for which a peak of an η-Zn phase (00·2) plane was detected byX-ray diffraction of the surface of a heat treated rectangular tube weredetermined to have an η phase.

4) Surface Roughness Ra

The surface roughness Ra of the coating layer of a heat treatedrectangular tube was measured in accordance with the specificationsprescribed by JIS B 0601 using a SURFCOM instrument manufactured byTokyo Seimitsu Co., Ltd. with a cut-off value of 0.8 mm. The measuringapparatus used was a SURFCOM 1900DX, and the probe (contact needle) wasa model E-DT-SS01A. At the time of measurement, roughness referencespecimens (E-MC-S24B manufactured by Tokyo Seimitsu Co., Ltd.) wereused, and it was confirmed that the measured value of the surfaceroughness Ra of the reference specimen with Ra 3.18 μm was in the rangeof ±2% of the nominal roughness (3.12-3.24 μm), and the measured valueof the surface roughness Ra of the reference specimen with Ra 0.41 μmwas in the range of 0.38-0.44 μm.

TABLE 2 Rate of Peak temp. Holding metal Roughness increase time temp. ηRa Mark (° C./sec) (sec) reached Appearance* phase (μm) Remark Range ofAt least at most at least yes At most 2.0 invention 300 2.0 Ac₃ 311-2532 0.7 1024 Circle yes 1.64 Inventive 311-4 419 1.6 1066 Triangle no2.83 Compar. 313-7 535 3.9 942 X no 3.48 Compar. 312-6 201 1.2 1066 X no3.00 Compar. 312-13 61 2.0 975 X no 2.47 Compar. 312-15 64 2.2 1029 X no4.04 Compar.

FIG. 3 shows graphs of X-ray diffraction strength, and FIG. 4 shows SEMimages of the surface and cross sections.

From the graphs in FIG. 3, it can be seen that an η-Zn peak was detectedin the example of the present invention but was not detected in acomparative example in which only ZnO and α Fe were detected.

As shown in Table 2 and FIG. 4, by satisfying the conditions prescribedby the present invention, it is possible to manufacture a bent metalmember having a Zn-based layer with a good appearance.

EXAMPLE 2

A heat treatment test was carried out in the same manner as inExample 1. The material being treated and the working and heat treatmentapparatus were the same as in Example 1. In Example 2, thermocoupleswere attached to the same steel tube at four locations in thecircumferential direction, and the surface condition at each locationwas investigated.

In addition to the categories for investigating the surface conditionused in Example 1, the following category was investigated by thefollowing method.

(Coating Composition)

A heat treated round tube with a coating layer was immersed in anaqueous 10% hydrochloric acid solution to which an inhibitor was added(1 gram per liter of 700 BK manufactured by Asahi Chemical Co., Ltd.) todissolve the coating layer, and the resulting solution was analyzed byICP spectral analysis and atomic absorption to determine the coatingweight and composition of the coating layer. The results are shown inTable 3.

In this measurement method, Zn oxides present on the coating layer andscale and a Fe—Zn solid solution phase which may present in admixturewith the coating layer dissolve together, so they are reflected in themeasured values but are not excluded from the values.

TABLE 3 Rate of Peak temp. Holding metal Coating Roughness increase timetemp. weight Fe % η Ra Mark (° C./sec) (sec) (° C.) (g/m²) (wt %) phase(μm) Remark Range of At least at most at least yes At most invention 3002.0 Ac₃ 2.0 392-3 530 0 789 54.5 13.5 yes 0.69 Compar. 392-4 530 0.242826 55.9 13.5 yes 0.78 Inventive 392-5 530 0.35 869 53.4 13.4 yes 0.84Inventive 392-6 530 0.402 896 50.0 12.4 yes 1.00 Inventive 392-8 5300.498 943 50.1 11.9 yes 1.96 Inventive 392-9 530 0.624 994 34.8 14.5 yes2.80 Compar. 392-17 530 0.778 1013 23.4 17.2 yes 3.46 Compar. 392-15 5300.816 1119 8.5 89.1 yes 2.45 Compar.

From the results shown in Table 3, it can be seen that the examples ofthe present invention has a prescribed coating weight.

1. (canceled)
 2. A bent metal member having a body formed from a tubularmetal material and having a shape which is bent two-dimensionally orthree-dimensionally, characterized in that the surface of the body has aZn-based coating which contains an η phase and which satisfies thefollowing Condition 2: Condition 2 Coating weight: at most 90 g/m² perside Fe content: 8-35 mass % Surface roughness Ra prescribed by JIS B0601: at most 2.0 μm.
 3. A bent metal member as set forth in claim 2having a closed cross section, an open cross section, or a shaped crosssection.